NOx emissions from automobiles, fossil fuel-fired power stations and industrial plants, and the like, need to be monitored to control and maintain low NOx emission levels to meet stringent NOx emission standards. For gasoline engines that run at a stoichiometric mixture of air and fuel, the engine out NOx emissions are generally controlled by three-way-catalysts (TWCs), which convert CO, hydrocarbons, and NOx simultaneously with high efficiency. For internal combustion engines that burn a lean mixture of fuel and air, however, the conventional TWCs do not reduce NOx efficiently. For lean burn NOx emission control, several other technologies are being investigated including lean NOx trap (LNT) aftertreatment systems and selective catalytic reduction (SCR) of NOx with urea. In an LNT system, also called a NOx adsorber catalyst (NAC), NOx is trapped on the catalyst during normal lean operations, and when the LNT reaches a certain NOx storage level, the engine is operated rich for a short period of time to cause the absorbed NOx to desorb from the catalyst surface. The rich exhaust gases contain CO and unburned hydrocarbons that reduce NOx to N2. In a urea-SCR system, a controlled amount of urea is injected to the exhaust which decomposes to form NH3, and NOx is selectively reduced by the NH3. In both LNT and SCR systems, accurately monitoring NOx concentration and NOx flux is important in order to achieve high NOx reduction efficiency.
Some commercially available NOx sensors use O2 sensing technologies and are not direct NOx measurement devices. These O2 sensing technologies suffer from problems associated with interference from other compounds that may exist in the exhaust gases. Therefore, new technologies for direct NOx measurements are preferable for more accurate and precise monitoring of NOx concentration and NOx flux. One approach is to apply catalytic components onto sensors that consist of interdigital electrodes, a heater, and temperature sensors. By measuring the change of the electrical properties (e.g. electrical impedances) of the catalytic components, it is feasible to directly gauge the NOx loading status on the catalyst. For example, WO9810272A1 describes a method for determining the NOx storage load of a NOx storage catalyst by using a monitoring sensor where the storage material forms the sensitive element in the sensor. U.S. Pat. No. 6,833,272 describes a sensor for determining the storage-state of an NH3-adsorbing SCR catalyst on the basis of sensing the electrical impedance of the SCR catalyst. SAE 2008-01-0447 discusses the monitoring of the NOx storage and reduction process, degree of NOx loading, thermal aging and sulphur poisoning via the measurement of electrical impedance of NAC coated sensors inserted in the NAC catalyst.
One of the NOx storage components reported in the literature (e.g. SAE 2008-01-0447) is a barium-based formulation, containing noble metals and ceria. It appears that the response of its electrical property to NOx loading was not strong and quickly approached a plateau. For example, at 350° C., the impedance only changed by about 6% and reached a plateau in about 2 minutes with a gas concentration of 0.055% NO (see FIG. 3 in SAE 2008-01-0447). In addition, the barium-based NOx storage material suffered from low NOx storage capacity at high temperatures.